Photodiodes formed on a thermally conductive layer and , photodiode systems

ABSTRACT

A photodiode device and method of manufacturing the same are disclosed. A stack of functional layers of the photodiode device, formed of crystalline semiconductor material, may be formed on a highly thermally conductive substrate, such as diamond or SiC. The stack of functional layers may be in contact with or close proximity to the thermally conductive substrate to thereby provide an efficient thermal conductive path between the functional layers and an external source, thereby mitigating problems that may result from overheating the photodiode device.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application is a continuation of U.S. patentapplication Ser. No. 15/909,598 filed Mar. 1, 2018, which claimspriority to U.S. Provisional Application No. 62/465,179, filed Mar. 1,2017 and U.S. Provisional Application No. 62/486,475, filed Apr. 18,2017, the contents of each of which being incorporated by reference intheir entirety.

TECHNICAL FIELD

Example embodiments of the present disclosure relate to a novelphotodiode and a novel method of manufacturing a photodiode.

RELATED ART

High power photodiode applications continue to be developed to andprovide novel solutions to address various challenges. For example,antenna arrays being driven by high power photodiodes are beingdeveloped. However, to power various systems to a desired level, thepower applied to a photodiode should be increased. For example, a lightbeam (e.g., laser light) may be modulated and used to drive aphotodiode, with the photodiode converting the light to an RF electricalsignal (e.g., to drive a corresponding RF antenna). However, heat isquickly generated by the photodiode in such an operation. Withoutefficient heat dissipation, problems associated with thermal failure orsaturation due to over-heating may occur.

A conventional photodiode structure may include a diode structure formedfrom several functional semiconductor layers on a semiconductorsubstrate, such as InP (indium phosphide). However, as InP has arelatively low thermal conductivity (e.g., 68 W/mK), heat dissipationthrough the InP substrate is often insufficient for many applications.

One approach to assist in dissipating heat from a photodiode is to use athermally conductive substrate to which the photodiode is flip-chipbonded. Heat generated by the photodiode may then be dissipated byproviding a heat dissipation path through the anode/cathodes of thephotodiode to conductive structure on the thermally conductivesubstrate. For example, “Improved power conversion efficiency inhigh-performance photodiodes by flip-chip bonding on diamond” by Xie etal. (Vol. 1, No. 6, Optica, December 2014) describes bonding aphotodiode to a diamond submount to improve heat dissipation of the heatgenerated by the photodiode. However, although heat dissipation isimproved with this approach, more efficient heat dissipation is stilldesired for many high-power photodiode applications to address problemsassociated with thermal failure or saturation due to over-heating.

SUMMARY

Disclosed herein are photodiode devices and methods of manufacturing thesame. In some embodiments, a photodiode device comprises a firstthermally conductive layer, a first stack of functional semiconductorlayers on the first thermally conductive layer, the first stack offunctional layers comprising an n-type semiconductor layer forming acathode and a p-type semiconductor layer forming an anode, the firststack of functional layers forming a first photodiode semiconductorstructure, a first conductive metal connection contacting the n-typesemiconductor layer, and a second conductive metal connection contactingthe p-type semiconductor layer. The bottommost layer of the first stackof functional semiconductor layers may be in contact with or close to anupper surface of the first thermally conductive layer. For example, thebottommost layer of the first stack of functional semiconductor layersmay spaced apart from the upper surface of the first thermallyconductive layer by no more than 7 microns.

Methods of manufacturing photodiode devices are also disclosed. In someembodiments, a method of manufacturing a photodiode device, comprisesforming a first photodiode semiconductor structure attached to an uppersurface of a first thermally conductive at a bottommost layer of thephotodiode semiconductor structure, the bottommost layer of the firstphotodiode semiconductor structure and the upper surface of the firstthermally conductive being in contact or spaced apart by a smalldistance, such as being separated by no more than 7 microns. Forming thefirst photodiode semiconductor structure may include epitaxially growinga plurality of crystalline material layers, the plurality of crystallinematerial layers comprising an n-type semiconductor material layer, anintrinsic semiconductor material layer and a p-type semiconductormaterial layer and patterning the plurality of crystalline materiallayers to form the first photodiode semiconductor structure.

Other novel improvements disclosed herein may be appreciated from thefollowing detailed disclosure.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects and features are reflected in the exemplaryembodiments set forth herein, with reference to the accompanying figuresin which:

FIGS. 1A to 1H illustrate an exemplary process of manufacturing aphotodiode according to an embodiment of the invention and resultingphotodiode structures.

FIGS. 2A to 2C illustrate an alternative process for forming a diamondbacked photodiode and a diamond sandwiched photodiode.

FIGS. 3A and 3B show details representing an alternative process to thatdisclosed with respect to the embodiment of FIGS. 1A to 1H.

FIGS. 4A and 4B illustrate an alternative embodiment in which a diamondbacked photodiode 200 is formed by epitaxially growing the stack offunctional layers on a diamond substrate.

DETAILED DESCRIPTION

The present disclosure now will be described more fully hereinafter withreference to the accompanying drawings, in which various exemplaryembodiments are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as limited to the exemplaryembodiments set forth herein. These example exemplary embodiments arejust that—examples—and many embodiments and variations are possible thatdo not require the details provided herein. It should also be emphasizedthat the disclosure provides details of alternative examples, but suchlisting of alternatives is not exhaustive. Furthermore, any consistencyof detail between various exemplary embodiments should not beinterpreted as requiring such detail—it is impracticable to list everypossible variation for every feature described herein. The language ofthe claims should be referenced in determining the requirements of theinvention.

Ordinal numbers such as “first,” “second,” “third,” etc. may be usedsimply as labels of certain elements, steps, etc., to distinguish suchelements, steps, etc. from one another. Terms that are not describedusing “first,” “second,” etc., in the specification, may still bereferred to as “first” or “second” in a claim. In addition, a term thatis referenced with a particular ordinal number (e.g., “first” in aparticular claim) may be described elsewhere with a different ordinalnumber (e.g., “second” in the specification or another claim).

It will be understood that when an element is referred to as being“connected” or “coupled” to or “on” another element, it can be directlyconnected or coupled to or on the other element or intervening elementsmay be present. In contrast, when an element is referred to as being“directly connected” or “directly coupled” to another element, or as“contacting” or “in contact with” another element, there are nointervening elements present.

As used herein, components described as being “electrically connected”are configured such that an electrical signal can be transferred fromone component to the other (although such electrical signal may beattenuated in strength as it transferred). Moreover, components that are“directly electrically connected” share a common electrical node throughelectrical connections by one or more conductors, such as, for example,wires, pads, internal electrical lines, through vias, etc. As such,directly electrically connected components do not include componentselectrically connected through active elements, such as transistors ordiodes.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe positional relationships, such as illustrated in the figures,e.g. It will be understood that the spatially relative terms encompassdifferent orientations of the device in addition to the orientationdepicted in the figures.

Terms such as “same,” “equal,” “planar,” or “coplanar,” as used hereinencompass near identically including variations that may occur, forexample, due to manufacturing processes. The term “substantially” may beused herein to emphasize this meaning, unless the context or otherstatements indicate otherwise.

FIGS. 1A to 1G illustrate an exemplary process of manufacturing aphotodiode according to an embodiment of the invention and the resultingphotodiode. As shown in FIG. 1A, one or more photodiodes 100 may beprovided. The photodiode 100 may be formed using conventionalsemiconductor manufacturing processes. For example, on a semiconductorsubstrate 10, a stack of doped semiconductor functional layers 20 may beepitaxially grown.

Semiconductor substrate 10 may be referred to as the photodiodesubstrate 10 in this disclosure. For example, substrate 10 may be asemi-insulating InP (SI InP) and may comprise a crystallinesemiconductor InP (indium phosphide) wafer lightly doped, such as with ap-type (acceptor) impurity (e.g., Fe (iron) or Zn (zinc)) or with bothn-type (donor) (e.g., Si (silicon) or Te (tellurium)) and p-typeimpurities.

The stack of semiconductor functional layers 20 may comprise an n+ InPlayer 20 a, an intrinsic InP layer 20 b, an intrinsic InGaAs (IndiumGallium Arsenide) layer 20 c and a p+ InGaAs layer 20 d. The height ofthe stack 20 may be a few microns or less, such as 3 microns or less. Asconventional, “n+” indicates the doping of a layer with n-type chargecarrier impurities (donor type) and “p+” indicates the doping of asemiconductor material with p-type charge carrier impurities (acceptortype) (with the “+” indicating a relatively higher concentration). Thus,the stack of doped semiconductor functional layers 20 forms a photodiodesemiconductor structure. The photodiode semiconductor structure formedby the stack of functional layers 20 extends from a cathode formed by n+InP layer 20 a to an anode formed by p+ InGaAs layer 20 d. A depletionregion is formed between the cathode and anode, and when exposed tolight, a photon may generate an electron-hole pair to cause aphotocurrent and voltage across the anode and cathode (in this example,20 a, 20 d). The photodiode semiconductor structure may take many forms,including conventional PN, PIN, UTC, MUTC, etc. Each layer of the stackof semiconductor functional layers 20 may be epitaxially grown and maybe a crystalline semiconductor layer. The p-type and n-type impuritiesmay be formed in situ during the epitaxial growth of the impurity dopedsemiconductor layer (e.g., 20 a or 20 d) of the stack of functionallayers 20. It will be appreciated that the stack of doped semiconductorfunctional layers 20 may include additional semiconductor layers in thestack of functional layers 20 and/or be formed with other types ofsemiconductor materials other than InP and InGaAs as described in thisexample.

The material layers of the stack of functional layers 20 (e.g., thematerial layers forming 20 a, 20 b, 20 c and 20 d) may be conformallyformed across the entire semiconductor substrate 10 (e.g., across anentire wafer forming the semiconductor substrate 10) and the stack offunctional layers 20 of the photodiode 100 may be formed by patterningthe material layers (such as by selective etching using aphotolithographically patterned photoresist as a mask) to expose the n+InP layer 20 a. It will be appreciated that other semiconductormaterials may be used to form the stack of doped semiconductorfunctional layers 20 to form a photodiode semiconductor structure. Itshould also be understood that a photodiode semiconductor structureincludes both the more conventional P-I-N photodiodes as well asphotodiodes formed with a stack of p-type doped, intrinsic and n-typedoped semiconductor layers. For example, a UTC (unitraveling carrier)photodiode and a MUTC (modified unitraveling carrier) photodiode arealso considered P-I-N photodiodes in this application.

Then, for each photodiode 100, a cathode electrode 32 may be formed onthe exposed n+ InP layer 20 a and an anode electrode 34 may be formed onthe p+ InGaAs layer 20 d, e.g., using an electroplating process. Thecathode electrode 32 and anode electrode 34 may each be formed of anelectrically conductive metal that also has a relatively high thermalconductivity, such as Au (gold), Ag (silver), Cu (copper), Al (Aluminum)or allows thereof. The electrodes 32 and 34 may each be formed as asingle continuous electrode or formed as several discrete, spaced apartsub-electrodes. For example, cathode electrode 32 may be formed to havea ring shape (from a top down perspective) to surround the stack ofsemiconductor functional layers 20. By forming the electrodes 32 and 34to contact and cover a large area of the semiconductor layer of thestack 20 (here, the corresponding one of 20 a and 20 d), a largerthermal conductive path may be formed between the photodiode 100 and thediamond submount on which the photodiode 100 is later mounted in thisexample (as discussed below).

Although only two photodiodes 100 are shown as being formed in FIG. 1A,it will be appreciated that additional photodiodes 100 may be formed atthe same time with the same processes on the same photodiode substrate10 (e.g., on the same wafer substrate). The photodiodes 100 may beisolated from each other by isolating the stack of semiconductorfunctional layers 20, such as by separating semiconductor layer 20 ainto discrete “islands” (e.g., via selective etching using a patternedphotoresist as a mask) on the photodiode substrate 10, eachcorresponding to a separate photodiode 100. The plurality of photodiodes100 may be formed in a two-dimensional array across a wafer (e.g.,across an SI InP wafer when SI InP is implemented as the photodiodesubstrate 10). For ease of explanation, the process will be describedwith respect to one photodiode 100, but it will be understood that theprocess and resulting structure applies equally to the simultaneousformation of a plurality of photodiodes 100.

After forming one or more photodiodes 100, a carrier substrate 60 isattached to the active surface of the structure of FIG. 1A with anadhesive 62. As shown in FIG. 1B, the adhesive 62 contacts the stack ofsemiconductor functional layers 20, cathode electrode 32 and anodeelectrode 34 on one side and the carrier substrate 60 on the other side.The carrier substrate 60 may comprise a silicon wafer, e.g., or othersubstrate, preferably suitable for conventional semiconductor processingand handling.

Then, as shown in FIG. 1C, at least part of the photodiode substrate 10is removed to thin or completely remove the photodiode substrate 10.Specifically, all or some of the backside of the photodiode substrate 10(opposite to the active surface of the structure of FIG. 1A) is removed,such as by grinding (e.g., lapping), etching (dry or wet etching), by acombination of grinding and etching (e.g., via chemical mechanicalpolishing (CMP)), etc. In FIG. 1C a portion of the photodiode substrate10 remains after this thinning process (the portion denoted withreference numeral 10′). The thinned photodiode substrate 10′ may have athickness of several microns or less, such as 5 microns or less or 2microns or less, e.g. The carrier substrate 60 may be used to hold thestructure of FIG. 1B in place, such as by placing the carrier substrate60 on a chuck of the CMP tool.

Alternatively, all of the photodiode substrate 10 may be removed duringthe thinning process while functional layers 20 remain after thethinning process. To facilitate removal of all of the photodiodesubstrate 10, prior to epitaxially growing functional layers 20 on thephotodiode substrate 10 as described with respect to FIG. 1A, anetch-stop layer (not shown) may be formed on the photodiode substrate10, and then the semiconductor functional layers 20 may be grown on theetch-stop layer (so that the etch stop layer is sandwiched between thephotodiode substrate 10 and the semiconductor functional layers 20). Theetch-stop layer may be epitaxially grown and be a crystallinesemiconductor layer, such as crystalline InGaAs, InGaAsP or AlAs. Uponthinning the photodiode substrate 10 (e.g., backside grinding of the InPwafer, chemical mechanical polishing, dry or wet etching,lapping/polishing), the end of the thinning process may be determinedupon detecting that the grinding/etching has reached the etch-stoplayer. For example, during the thinning process, the residue materialremoved from the backside of the photodiode substrate 10 may becontinually sampled to detect the material of the etch-stop layer (orany resulting compound due to chemical reactions with an etchant) andwhen the etch-stop layer is detected, the thinning process may bestopped. Although the remaining figures and description regarding thismanufacturing process are described with respect to a thinned photodiodesubstrate 10′ remaining after the thinning process, it will beappreciated that the process is equally applicable to a full removal ofthe photodiode substrate 10, with the resultant structure beingequivalent (e.g., the same except with complete removal of photodiodesubstrate 10 and with the possible addition of an etch-stop layer asdescribed herein).

As shown in FIG. 1D, a diamond wafer 40 is attached with adhesive 42 tothe backside of the photodiode structure resulting from the thinningprocess of FIG. 1C to provide a diamond backed photodiode 200. Theadhesive 42 may be coated either to the diamond wafer 40 or to thesurface of the backside of the FIG. 1C structure (e.g., to the thinnedphotodiode substrate 10′, the etch-stop layer (not shown) or to theoutermost one of the exposed functional layer, here semiconductor InPlayer 20 a). Then, the diamond wafer 40 may be joined and adhered to thefunctional layers 20 (with or without a thinned photodiode substrate10′). Adhesive 42 may have a thickness less than two microns, such asless than a micron and may be transmissive to light (such as toinfrared, visible and/or ultraviolet light), such as a transparentpolymer material such as BCB (benzocyclobutene) or SU-8.

The diamond wafer 40 has a high thermal conductivity (˜2000 W/mK ormore) and replaces or substantially replaces the low-thermalconductivity InP substrate (68 W/mK). The thickness of the diamond wafer40 may be made large (e.g., 200 microns or more, such as greater than 2mm) to increase the heat transmission path capacity. In addition, thediamond material can transmit a large spectrum of light includinginfrared, visible and ultraviolet spectrums of light. Transmission ofinfrared light is particularly useful in order to allow transmission oflight provided by readily available optic fibers (often transmittinginfrared light of around 850, 1300 and 1550 nm). Thus, light may betransmitted through the diamond wafer 40 and impinge on the functionallayers 20 which then convert the light to an electrical signal(providing a voltage output on the cathode electrode 32 and anodeelectrode 34).

The diamond wafer 40 is also advantageous to use with an RF (radiofrequency) antenna and/or RF antenna array as the RF signals generatedby the antenna(s) may be transmitted through the diamond wafer 40. Thediamond wafer 40 avoids substantial interference with these RF signals.

As shown in FIG. 1E, the carrier substrate 60 and adhesive 62 may beremoved from the diamond backed photodiode 200, such as by detachingwith physical force, using a suitable solvent to dissolve or weakenadhesive layer 62 or by applying heat or ultraviolet light to weaken theadhesive layer 62 and detaching by peeling the carrier substrate 60 awayfrom the diamond backed photodiode 200.

As shown in FIG. 1F, a diamond submount may be prepared. For example,the diamond submount may comprise a diamond wafer 50 having a metalpattern 52 formed at least one of its surfaces. The diamond wafer 50 mayhave any of the features described with respect to diamond wafer 40,including having a thickness of 200 microns or more, such as greaterthan 2 mm.

The metal pattern 52 may be formed by any suitable conductor, such asAu, Ag, Cu, Al, W, or alloys thereof. The metal pattern 52 may be formedper the design of the system in which the diamond backed photodiode 200is to be used. For example, the metal pattern 52 may include elements 52a and 52 b forming metal pads for respectively connecting to the cathodeelectrode 32 and anode electrode 34 of the diamond backed photodiode200. Discrete wiring may also comprise portions of the metal pattern 52to transmit electrical signals received from the diamond backedphotodiode 200 to an appropriate source. In some examples, the metalpattern 52 may include antennas, such as dipole antennas or patchantennas. See, for example, U.S. Provisional Application No. 62/465,181filed Mar. 1, 2017 and U.S. patent application Ser. No. 15/242,459 filedAug. 19, 2016 both of which are incorporated by reference in theirentirety, teaching various antennas and antenna arrays that are drivenand/or controlled by photodiodes, their operation, as well as exemplaryphysical and structural relationships between the antennas, photodiodesand optical feeds (e.g., optical waveguides) to the photodiodes. In someexamples, elements 52 a and 52 b may form portions of an antenna (e.g.,radiating arms of a dipole antenna). In some examples, the antennas maybe formed of an additional metal pattern (not shown) on the oppositeside of the diamond wafer 50 (external side of diamond wafer 50 awayfrom the diamond backed photodiode 200) and be connected to theelectrical RF signals provided by the diamond backed photodiode 200 viapads 52 a, 52 b and wiring (e.g., through vias in the diamond backedphotodiode 200—not shown) or RF waveguides (e.g., microstrip—not shown).

Additional layers may be provided to facilitate the design of thedesired system, such as dielectric layers on one or both sides of thediamond wafer 50 and additional layers of patterned metal. In someexamples, antennas described herein may be formed of such an additionalmetal pattern on one of these additional dielectric layers.

The metal pattern 52 may be formed by electroplating, by a damasceneprocess or by etching a metal layer deposited on the surface of thediamond wafer 50. In the electroplating process, a thin conductive seedlayer may be deposited across the surface of the diamond wafer, aninsulating layer may then be formed on the seed layer and patterned toform openings in the insulating layer exposing the seed layer. Then, thestructure may be subjected to electroplating by depositing the diamondwafer 50 in an electroplating bath to deposit metal of the bath into theopenings of within the insulating layer (by applying a voltage to theconductive seed layer, the portions of the seed layer exposed by theopenings of the insulating layer will attract and have attached metalsuspended in the bath). The patterned insulating layer may then beremoved and an etch may be performed to remove the exposed seed layer onwhich patterned insulating layer had covered, leaving the electroplatedmetal as the metal pattern 52. The metal pattern 52 may also be formedwith a damascene process which may comprise depositing metal over andwithin openings of a patterned insulating layer formed on the diamondwafer 50, planarizing the metal to expose the upper surface of thepatterned insulating layer and removing the insulating layer to leavethe metal in the openings as the metal pattern 52. The metal pattern 52may also be formed by directly patterning a metal layer deposited on thesurface of the diamond wafer 50 (e.g., by selectively etching using apatterned photoresist formed over the metal layer as a mask).

As shown in FIG. 1G, the diamond backed photodiodes 200 may be mountedon the diamond submount by connecting the cathode electrode(s) 32 to thepad(s) 52 a and the anode electrode(s) 34 to pad(s) 52 b. As describedwith respect to the cathode and anode electrodes 32, 34, the pads 52 aand 52 b may have a variety of configurations. For example, in thecross-sectional view of FIG. 1G, pad metal 52 a may be part of a largecontinuous portion of the metal pattern 52 or may be discretely formedpads (relatively larger areas of metal with respect to a top down view)connected by wiring of the metal pattern 52 (relatively smaller width ascompared to the pad area dimensions).

In some examples, the metal pattern 52 or an additional metal pattern 52may form antennas at a location other than that shown in FIGS. 1G and1H. For example, antennas (to which corresponding diamond backedphotodiodes 200 are electrically connected) may be formed on a substrateor other layer on which the stack of functional layers are formed, suchas on thinned photodiode substrate 10′, or on the upper or lower surfaceof diamond wafer 40, or on the lower surface of diamond wafer 50. U.S.patent application Ser. No. 15/909,798, filed Mar. 1, 2018, entitled“TWO-DIMENSIONAL CONFORMAL OPTICALLY-FED PHASED ARRAY AND METHODS OFMANUFACTURING THE SAME” by Shouyuan Shi, Dennis Prather, Peng Yao andJanusz Murakowski (attorney docket no. PSI-113) (a Non-Provisional ofU.S. Provisional Application No. 62/465,181, filed on Mar. 1, 2017, U.S.Provisional Patent Application No. 62/590,066, filed Nov. 22, 2017) isincorporated by reference for providing further exemplary details ofvarious alternative antenna and photodiode formation and theirarrangements, as well as alternative interconnections with betweenantenna-photodiode pairs, as well as use of different antenna types andthe use of a signal feed module (which may include a plurality ofoptical waveguides integrally formed therein or include a plurality ofRF waveguides integrally formed therein), such as with diamond wafer 40or 50. This concurrently filed application is also incorporated byreference for exemplary details of an antenna array and an antenna arraymodule, with which the embodiments described herein may implemented at asystem level.

Mounting the diamond backed photodiodes 200 to the diamond submount (50,52) may be performed with conventional flip chip mounting techniques.For example, conductive bumps 54 may be placed on pads 52 a, 52 b andthe diamond backed photodiode by be placed to have the cathode electrode32 and anode electrode 34 contact the conductive bumps 54 (e.g., goldbumps such as gold balls). A metal reflow process may be performed toheat the conductive bumps 54, causing at least part of the bumps 54 tomelt and merge with metal of the corresponding cathode electrode 32,anode electrode 34 and pads 52 a, 52 b. Upon cooling, such mergedconductive material may form a vertically extending conductive metalconnection (e.g., a conductive metal pillar) which provides a directelectrical and physical connection between the stack of functionallayers 20 and diamond submount (e.g., to radiating arms of dipoleantennas or wiring formed by the patterned metal on the diamondsubmount). It should be appreciated that the metal connections betweenthe stack of functional layers 20 and the diamond submount (50, 52) maybe implemented in a variety of ways. For example, although the abovedescription indicates formation of an anode electrode 34 on the stack offunctional layers 20, such anode electrode 34 may be omitted and aconductive bump 54 may contact the functional layers 20 to connect thefunctional layers 20 to the diamond submount and the electricalcomponents thereon (e.g., antennas and/or wiring as described herein).

In addition, it should be appreciated that connecting the diamondsubmount (50, 52) to the diamond backed photodiode 200 may be done atthe wafer level. For example, a two-dimensional array of diamond backedphotodiodes 200 (arrayed on the same diamond wafer 40) may be connectedto the same diamond submount (e.g., patterned metal 52 on the samediamond wafer 50) to thereby simultaneously mount and connect aplurality of diamond backed photodiodes 200 to the diamond submount (50,52). For example, a two-dimensional array of diamond backed photodiodes200 may be connected to a two-dimensional array of antennas, such asdipole antennas or patch antennas, formed on the diamond submount, whereeach of the antennas has a location on the diamond submount (50, 52)corresponding to one of the diamond backed photodiodes, creating aplurality of diamond backed photodiode—antenna pairs. Alternatively,mounting of each diamond backed photodiode 200 may be performedindividually by separating the diamond backed photodiodes 200 from oneanother (e.g., through a laser cutting process) and then attaching eachdiamond backed photodiode 200 to the diamond submount (50, 52).

While the figures illustrate a relatively close spacing of neighboringdiamond backed photodiodes 200, in some implementations the diamondbacked photodiodes 200 may be spaced apart from neighboring photodiodes200 by a significant distance. For example, spacing between neighboringphotodiodes 200 (e.g., arranged in a two-dimensional array on the thinphotodiode substrate 10′) may have be at least five times or more thanthe width of the photodiode 200 (width referring to the maximum width,which in this embodiment may correspond to the width of functional layer20 a). For example, with respect to the vertical cross-sectional view ofFIGS. 1G and 1H, the distance between neighboring photodiodes 200 may bemore than 10 times W, where W is the width of functional layer 20 a).Providing such additional spacing may be helpful to reduce the effectsof heat generated from neighboring photodiodes 200.

The length of the conductive connections (e.g., cathode electrode 32,conductive bump 54 and any wiring formed by patterned metal 52 toconnect to the antenna radiating arm, or similar conductive connectionto connect to the anode) may be made less than one half the wavelengthof the RF electromagnetic signal (e.g., corresponding to the RF carrierfrequency) to be emitted by the antenna radiating arm, avoiding the useof an RF waveguide and baluns. The length of the antenna radiating armmay be one half the wavelength of this RF electromagnetic signal (e.g.,corresponding to the RF carrier frequency). However, it should beappreciated that other electrical connections between the diamond backedphotodiode 200 and corresponding electrical structure may be used, whichmay not involve use of the diamond submount (50, 52) (i.e., the diamondsubmount (50, 52) is optional and may be avoided altogether in someimplementations).

FIG. 1H illustrates the resulting structure according to the process ofthis embodiment when thinning the photodiode substrate 10 as describedwith respect to FIG. 1C results in full removal of the photodiodesubstrate 10.

As will be appreciated, providing a layer of diamond in contact with orspaced apart by a small distance from the functional layers 20 of thephotodiode provides a significant thermal transfer path from thefunctional layers 20 of the photodiode to an external source. Forexample, the spacing between the surface of the diamond substrate 40 andthe opposing surface of functional layer 20 a may be seven microns orless (e.g., separated by a 2 micron thick (or less) adhesive layer 42and a five micron thick (or less) thinned photodiode substrate 10′).However, such separation may be made smaller, such as three microns orless, when using a thinner adhesive layer 42 (e.g., 1 micron or less), athinner photodiode substrate 10′ (e.g., 2 microns or less) and/or fullyremoving the photodiode substrate 10 during the thinning process.

FIGS. 2A to 2C illustrate an alternative process for forming a diamondbacked photodiode and a diamond sandwiched photodiode. Processes andstructure that may be the same as that described with respect to FIGS.1A to 1G may be omitted for brevity. In FIG. 2A, a photodiode substrate10 (e.g., SI InP) is bonded to a diamond wafer via adhesive 42. Then, asshown in FIG. 2B, the photodiode substrate 10 is thinned by etching,grinding, CMP, etc. forming a thinned photodiode substrate 10′ (e.g.,having a thickness of 5 microns or less, such as 2 microns or less. Onthe exposed surface of the thinned photodiode substrate 10′, the stackof functional layers 20 are formed and patterned, and cathode electrodes32 and anode electrodes 34 are formed as described herein with respectto FIG. 1A, resulting in the formation of a plurality of diamond backedphotodiodes 200 as shown in FIG. 2C. The remaining steps may be the sameas that described with respect to FIGS. 1F and 1G to obtain a pluralityof diamond sandwiched photodiodes.

FIGS. 3A and 3B show details representing an alternative process to thatdisclosed with respect to the embodiment of FIGS. 1A to 1H. In thisalternative embodiment, the diamond formed on the backside surface ofthe thinned photodiode substrate 10′ may be epitaxially grown, ratherthan attaching a diamond wafer with an adhesive. The process stepsdescribed with respect to FIGS. 1A to 1C may be the same in thisalternative embodiment. However, after thinning the photodiode substrate10 to obtain the thinned photodiode substrate 10′ as described withrespect to FIG. 1C, the exposed thinned photodiode substrate 10′ mayhave an epitaxial diamond layer 40′ formed thereon (i.e., by epitaxiallygrowing diamond layer 40′ on the photodiode substrate 10′) (as shown inFIG. 3A). In some examples, it may be beneficial to first epitaxiallygrow a strain buffer layer (not shown) of a crystalline semiconductormaterial having a lattice constant intermediate to the lattice constantsof the crystalline thinned photodiode substrate 10′ and the epitaxialdiamond layer 40′ (e.g., the crystalline buffer layer may have a latticeconstant between SI InP and diamond) to interpose a strain buffer layerbetween the crystalline thinned photodiode substrate 10′ and the diamondlayer 40′. Such a strain buffer layer may be the same as strain bufferlayer 22 described with respect to FIGS. 4A and 4B, but it will beappreciated that the order of formation of the relevant portions of thebuffer layer 22 should be reversed when the strain buffer layer isformed with a graded lattice constant or with sub-layers havingdifferent lattice constants as described below.

The epitaxial diamond layer 40′ may be grown to an appropriate thicknessto facilitate heat transfer from the diamond backed photodiode 200 to anexternal source, such as a metal heat sink attached to the diamondbacked photodiode. For example, the epitaxial layer 40′ may be grown tobe greater than 200 microns, such as 2 mm or more. When the diamondlayer 40′ is grown on a strain buffer layer that is formed on thethinned photodiode substrate 10′, it may be grown in the same epitaxialprocess as growing the strain buffer layer (e.g., in situ in the sameprocess chamber without vacuum break of the chamber pressure or removalof the photodiode structure).

After growing the epitaxial diamond layer 40′, the steps may be the sameas that described with respect to FIGS. 1E to 1G, resulting in thestructure shown in FIG. 3B.

FIGS. 4A and 4B illustrate an alternative embodiment in which a diamondbacked photodiode 200 is formed by epitaxially growing the stack offunctional layers forming the photodiode semiconductor structure on adiamond substrate. In this example, diamond substrate 40 is used as aseed layer on which to grow several epitaxial layers. As shown in FIG.4A, diamond substrate 40 may have formed there on a strain buffer layer22, a thin photodiode substrate 10″ and a stack of crystallinesemiconductor material layers 20 a′, 20 b′, 20 c′ and 20 d′ (whenpatterned form a stack of the semiconductor functional layers 20). Thestrain buffer layer 22, the thin photodiode substrate 10″ and each layer20 a′, 20 b′, 20 c′ and 20 d′ of the stack of semiconductor functionallayers 20 may be epitaxially grown in sequential order on the diamondsubstrate 40, each being formed as a crystalline material. The materialof the thin photodiode substrate 10″ and the semiconductor functionallayers 20 may be the same as described herein and may have additionallayers formed in the stack of functional layers 20, as described herein.It will be appreciated that the thin photodiode substrate 10″ and strainbuffer layer 22 may be made very thin when epitaxially grown on thediamond substrate 40. The total distance between the stack ofsemiconductor functional layers 20 and the surface of the diamondsubstrate may be less than 200 nm, for example.

The crystalline structure of the strain buffer layer 22 may have alattice constant intermediate to the lattice constants of thecrystalline thin photodiode substrate 10″ and the diamond substrate 40(e.g., the crystalline buffer layer may have a lattice constant betweenSI InP and diamond). For example, when the thin photodiode substrate 10″is a conventional semiconductor substrate (such as InP or InGaAs), thethin photodiode substrate 10″ may have a crystalline lattice constant ofabout 5.85 (e.g., 5.85+/−5% as InGaAs lattice constant may range fromabout 6.05 to about 5.65 depending on the selected relativeconcentrations of the atomic elements of In, Ga and As). The latticeconstant of the thermally conductive crystalline diamond substrate 40may be 3.567. Thus, the strain buffer layer 22 be formed of acrystalline material having a lattice constant between 3.6 and about5.8. In addition, the strain buffer layer 22 may be formed as severalcrystalline layers (e.g., sub-layers) that are sequentially grown insitu, with each subsequent crystalline strain sub-layer grown beingformed of a crystalline material having a lattice constant greater thanthe layer on which it was grown (e.g., an increasing lattice constant ofthe sub-layers of the strain buffer layer 22 in a direction away fromthe diamond substrate 40 to the photodiode substrate 10″).

For example, the epitaxial strain buffer layer 22 may be formed ofsilicon carbide (SiC) where sub-layers closer to diamond substrate 40are formed with a relatively lower percentage of Si and a higherpercentage of C as compared to those sub-layers further away from thediamond substrate 40 and closer to the photodiode substrate 10″. In someexamples, the epitaxial growth may continuously change a precursor gassupply to continuously increase the lattice constant of the strainbuffer layer crystalline material as it is epitaxially grown (e.g.,continuously increase a ratio of Si precursor gas to C precursor gassupplied to a CVD chamber that performs the epitaxial growth of thestrain buffer layer 22). The photodiode substrate 10″ may be epitaxiallygrown in the same chamber as the process of growing the epitaxial strainlayer and may be performed in situ (e.g., without a vacuum break).

The stack of crystalline semiconductor material layers 20 a′, 20 b′, 20c′ and 20 d′ may be patterned forming a stack of functional layers 20(e.g., 20 a, 20 b, 20 c and 20 d) for each formed photodiode 200 asdescribed herein (e.g., with respect to FIG. 1A) and cathode electrodes32 and anode electrodes 34 may be added as described herein to obtainthe diamond backed photodiodes 200 as shown in FIG. 4B. If desired,further processes may be performed, such as those described with respectto FIGS. 1F and 1G.

In the embodiments described herein, diamond has been implemented as thethermally conductive layer (e.g., substrate 40 or epitaxial layer 40′)in connection with the details of the process and device embodiments.However, other materials may be used rather than diamond. Similarly,other materials may be used for a strain buffer layer. The material ofthe thermally conductive layer may be selected based on its thermalconductivity and its transparency to the light that the photodiode isconfigured to generate a photocurrent (e.g., a voltage output on itsanode and cathode electrodes) upon exposure to such light. The followingmaterials may be alternative options for one or both of the thermallyconductive substrates and the strain buffer layer:

-   -   Silicon carbide (SiC) 3c, 4h, 6h—this may be preferable when the        photodiode senses ultraviolet light as certain SiC is        transparent to ultraviolet light.    -   Silicon carbide (SiC) in the form of moissanite—this may be        preferable when the photodiode senses visible or infrared light.    -   Graphene    -   Cubic boron arsenide

The thermal conductivity of such thermally conductive layers may begreater than 350 W/mK (Watts per meter per Kelvin), and greater than 500W/mK and preferably greater than 1000 W/mK.

It should be appreciated that the materials referenced herein withrespect to the described strain buffer layers and alternatives forreplacing the thermally conductive diamond substrates/submounts aredescribed as crystalline, it is possible for such crystalline materialsto include crystalline defects which may, in some circumstances, resultin separate crystal formation in the crystalline substrates and strainedbuffer layers. However, it will be appreciated that “crystalline” asused herein is not intended to reference polycrystalline or amorphousmaterials, although such materials may also be implemented with theembodiments described herein.

In some examples, it may be possible to use a photodiode substrate ofdiamond, graphene, boron arsenide, SiC, e.g., that are appropriatelydoped to form the photodiode semiconductor structure (either byimplanting of appropriate p dopants or n dopants to form the p dopedand/or n doped semiconductor layers of the photodiode semiconductorstructure, or by in-situ doping during epitaxial growth of theselayers). In this case, the photodiode substrate need not be thinned butmay embody the features of both the photodiode substrate and thethermally conductive substrate described herein. The submount(substrate) may be flip-chip bonded to the photodiode electrodes formedon this thermally conductive substrate as described herein.

It is emphasized that the descriptions herein are exemplary anddeviations from which will be recognized to fall within the scope of theinvention as set forth in the claims of this application.

1. A photodiode device comprising: a first thermally conductive layerformed of silicon carbide (SiC); a first stack of semiconductorfunctional layers on the first thermally conductive layer, the firststack of semiconductor functional layers comprising an n-typesemiconductor layer forming a cathode and a p-type semiconductor layerforming an anode, the first stack of semiconductor functional layersforming a first photodiode semiconductor structure; a first conductivemetal connection contacting the n-type semiconductor layer; and a secondconductive metal connection contacting the p-type semiconductor layer,wherein the bottommost layer of the first stack of semiconductorfunctional layers is in contact with or is separated from an uppersurface of the first thermally conductive layer by no more than 7microns.
 2. The photodiode device of claim 1, further comprising: aphotodiode substrate on which the first stack of semiconductorfunctional layers is formed; and an adhesive connecting the photodiodesubstrate to the first thermally conductive layer, wherein the adhesiveis transmissive to light.
 3. The photodiode device of claim 2, whereinthe adhesive is transmissive to infrared light.
 4. The photodiode deviceof claim 2, wherein the adhesive is one of BCB (benzocyclobutene) orSU-8.
 5. The photodiode device of claim 2, wherein the photodiodesubstrate has a thickness of 5 microns or less.
 6. The photodiode deviceof claim 5, wherein the photodiode substrate is a layer ofsemi-insulative indium phosphide (InP).
 7. The photodiode device ofclaim 6, wherein n-type semiconductor layer of the first stack ofsemiconductor functional layers is an epitaxial InP layer doped withn-type impurities and contacts the photodiode substrate.
 8. Thephotodiode device of claim 5, further comprising a plurality of stacksof semiconductor functional layers including the first stack ofsemiconductor functional layers, wherein the plurality of stacks ofsemiconductor functional layers are spaced apart from one another andformed on the photodiode substrate, and each of the plurality of stacksof semiconductor functional layers forming a corresponding photodiodesemiconductor structure to form a plurality of photodiode semiconductorstructures on the photodiode substrate, and wherein the photodiodesubstrate is an intrinsic semiconductor crystalline substrate.
 9. Thephotodiode device of claim 8 wherein the plurality of photodiodesemiconductor structures are arranged in a two-dimensional array on thephotodiode substrate.
 10. The photodiode device of claim 9, wherein thefirst photodiode semiconductor structure is spaced apart from each ofimmediate neighboring ones of the plurality of photodiode semiconductorstructures by a distance no less than 10 times the maximum width firstphotodiode semiconductor structure.
 11. The photodiode device of claim1, further comprising: an adhesive contacting and adhering the firstphotodiode semiconductor structure to the first thermally conductivelayer, wherein the adhesive is transmissive to light.
 12. The photodiodedevice of claim 11, wherein the adhesive contacts the n-typesemiconductor layer of the first stack of functional semiconductorlayers.
 13. The photodiode device of claim 1, further comprising: aphotodiode substrate having a first surface on and contacting a surfaceof the first thermally conductive layer and having a second surface,opposite to the first surface, on which the first stack of semiconductorfunctional layers is formed.
 14. The photodiode device of claim 1,further comprising: a buffer crystalline layer having a first surface onand contacting a surface of the first thermally conductive layer andhaving a second surface, opposite to its first surface; and a photodiodesubstrate having a first surface on and contacting the second surface ofthe buffer crystalline layer and having a second surface, opposite toits first surface, on which the first stack of semiconductor functionallayers is formed, wherein the buffer crystalline layer comprises acrystalline material having a lattice constant between a latticeconstant of the first thermally conductive layer and a lattice constantof the photodiode substrate.
 15. The photodiode device of claim 1,further comprising: a second thermally conductive layer; a conductivemetal pattern on a first surface of the second thermally conductivelayer in contact with the first conductive metal connection and thesecond conductive metal connection, and wherein the first photodiodesemiconductor structure is located between the first thermallyconductive layer and the second thermally conductive layer.
 16. Thephotodiode device of claim 15, wherein the first conductive metalconnection is electrically connected by to a first radiating arm of afirst dipole antenna, wherein the second conductive metal connection iselectrically connected to a second radiating arm of the first dipoleantenna, and wherein the first and second radiating arms of the firstdipole antenna are elements of the conductive metal pattern.
 17. Thephotodiode device of claim 16, wherein the first and second conductivemetal connection comprise conductive metal pillars extending between thefirst thermally conductive layer and the second thermally conductivelayer.
 18. The photodiode device of claim 17, further comprising: aplurality of stacks of semiconductor functional layers including thefirst stack of functional semiconductor layers, wherein the plurality ofstacks of semiconductor functional layers are spaced apart from oneanother on the first thermally conductive layer, and each of theplurality of stacks of semiconductor functional layers forming acorresponding photodiode semiconductor structure to form a plurality ofphotodiode semiconductor structures on the first thermally conductivelayer; and a plurality of dipole antennas, including the first dipoleantenna, each of the plurality of dipole antennas formed on the secondthermally conductive layer and being connected to a corresponding one ofthe photodiode semiconductor structures.
 19. The photodiode device ofclaim 18, further comprising a photodiode substrate formed of intrinsicsemiconductor crystalline material on which the plurality of stacks ofsemiconductor functional layers are formed.
 20. The photodiode device ofclaim 19, wherein the photodiode substrate has a thickness of 5 micronsor less. 21-39. (canceled)
 40. The photodiode device of claim 9, whereinthe first photodiode semiconductor structure is spaced apart from eachof immediate neighboring ones of the plurality of photodiodesemiconductor structures by a distance no less than 5 times the maximumwidth first photodiode semiconductor structure.
 41. The photodiodedevice of claim 15, wherein the second thermally conductive layer isformed of SiC.
 42. The photodiode device of claim 15, wherein the secondthermally conductive layer has a thermal conductivity greater than 350W/mK
 43. The photodiode device of claim 1, wherein the first thermallyconductive layer has a thermal conductivity greater than 350 W/mK. 44.The photodiode device of claim 1, wherein the first thermally conductivelayer is an epitaxial layer.
 45. The photodiode device of claim 1,wherein the first thermally conductive layer is crystalline.
 46. Thephotodiode device of claim 1, further comprising a strain buffer layerinterposed between the first thermally conductive layer and the firststack of semiconductor functional layers.
 47. The photodiode device ofclaim 46, wherein the strain buffer layer is a crystalline material witha graded lattice constant.
 48. The photodiode device of claim 47,wherein the crystalline material of the strain buffer layer is SiC. 49.The photodiode device of claim 48, wherein the strain buffer layer isformed of a plurality of sublayers.
 50. The photodiode device of claim48, wherein the lattice constant of the strain buffer layer increases ina direction away from the first thermally conductive layer.